/*  $OpenBSD: queue.h,v 1.46 2020/12/30 13:33:12 millert Exp $  */
/*  $NetBSD: queue.h,v 1.11 1996/05/16 05:17:14 mycroft Exp $   */

/*
 * Copyright (c) 1991, 1993
 *  The Regents of the University of California.  All rights reserved.
 *
 * Redistribution and use in source and binary forms, with or without
 * modification, are permitted provided that the following conditions
 * are met:
 * 1. Redistributions of source code must retain the above copyright
 *    notice, this list of conditions and the following disclaimer.
 * 2. Redistributions in binary form must reproduce the above copyright
 *    notice, this list of conditions and the following disclaimer in the
 *    documentation and/or other materials provided with the distribution.
 * 3. Neither the name of the University nor the names of its contributors
 *    may be used to endorse or promote products derived from this software
 *    without specific prior written permission.
 *
 * THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND
 * ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
 * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
 * ARE DISCLAIMED.  IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE
 * FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
 * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
 * OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
 * HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
 * LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
 * OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
 * SUCH DAMAGE.
 *
 *  @(#)queue.h 8.5 (Berkeley) 8/20/94
 */

#ifndef _SYS_QUEUE_H_
#define _SYS_QUEUE_H_

/*
 * This file defines five types of data structures: singly-linked lists,
 * lists, simple queues, tail queues and XOR simple queues.
 *
 *
 * A singly-linked list is headed by a single forward pointer. The elements
 * are singly linked for minimum space and pointer manipulation overhead at
 * the expense of O(n) removal for arbitrary elements. New elements can be
 * added to the list after an existing element or at the head of the list.
 * Elements being removed from the head of the list should use the explicit
 * macro for this purpose for optimum efficiency. A singly-linked list may
 * only be traversed in the forward direction.  Singly-linked lists are ideal
 * for applications with large datasets and few or no removals or for
 * implementing a LIFO queue.
 *
 * A list is headed by a single forward pointer (or an array of forward
 * pointers for a hash table header). The elements are doubly linked
 * so that an arbitrary element can be removed without a need to
 * traverse the list. New elements can be added to the list before
 * or after an existing element or at the head of the list. A list
 * may only be traversed in the forward direction.
 *
 * A simple queue is headed by a pair of pointers, one to the head of the
 * list and the other to the tail of the list. The elements are singly
 * linked to save space, so elements can only be removed from the
 * head of the list. New elements can be added to the list before or after
 * an existing element, at the head of the list, or at the end of the
 * list. A simple queue may only be traversed in the forward direction.
 *
 * A tail queue is headed by a pair of pointers, one to the head of the
 * list and the other to the tail of the list. The elements are doubly
 * linked so that an arbitrary element can be removed without a need to
 * traverse the list. New elements can be added to the list before or
 * after an existing element, at the head of the list, or at the end of
 * the list. A tail queue may be traversed in either direction.
 *
 * An XOR simple queue is used in the same way as a regular simple queue.
 * The difference is that the head structure also includes a "cookie" that
 * is XOR'd with the queue pointer (first, last or next) to generate the
 * real pointer value.
 *
 * For details on the use of these macros, see the queue(3) manual page.
 */

#if defined(QUEUE_MACRO_DEBUG) || (defined(_KERNEL) && defined(DIAGNOSTIC))
#define _Q_INVALID ((void *)-1)
#define _Q_INVALIDATE(a) (a) = _Q_INVALID
#else
#define _Q_INVALIDATE(a)
#endif

/*
 * Singly-linked List definitions.
 */
#define SLIST_HEAD(name, type)                      \
struct name {                               \
    struct type *slh_first; /* first element */         \
}

#define SLIST_HEAD_INITIALIZER(head)                    \
    { NULL }

#define SLIST_ENTRY(type)                       \
struct {                                \
    struct type *sle_next;  /* next element */          \
}

/*
 * Singly-linked List access methods.
 */
#define SLIST_FIRST(head)   ((head)->slh_first)
#define SLIST_END(head)     NULL
#define SLIST_EMPTY(head)   (SLIST_FIRST(head) == SLIST_END(head))
#define SLIST_NEXT(elm, field)  ((elm)->field.sle_next)

#define SLIST_FOREACH(var, head, field)                 \
    for((var) = SLIST_FIRST(head);                  \
        (var) != SLIST_END(head);                   \
        (var) = SLIST_NEXT(var, field))

#define SLIST_FOREACH_SAFE(var, head, field, tvar)          \
    for ((var) = SLIST_FIRST(head);             \
        (var) && ((tvar) = SLIST_NEXT(var, field), 1);      \
        (var) = (tvar))

/*
 * Singly-linked List functions.
 */
#define SLIST_INIT(head) {                      \
    SLIST_FIRST(head) = SLIST_END(head);                \
}

#define SLIST_INSERT_AFTER(slistelm, elm, field) do {           \
    (elm)->field.sle_next = (slistelm)->field.sle_next;     \
    (slistelm)->field.sle_next = (elm);             \
} while (0)

#define SLIST_INSERT_HEAD(head, elm, field) do {            \
    (elm)->field.sle_next = (head)->slh_first;          \
    (head)->slh_first = (elm);                  \
} while (0)

#define SLIST_REMOVE_AFTER(elm, field) do {             \
    (elm)->field.sle_next = (elm)->field.sle_next->field.sle_next;  \
} while (0)

#define SLIST_REMOVE_HEAD(head, field) do {             \
    (head)->slh_first = (head)->slh_first->field.sle_next;      \
} while (0)

#define SLIST_REMOVE(head, elm, type, field) do {           \
    if ((head)->slh_first == (elm)) {               \
        SLIST_REMOVE_HEAD((head), field);           \
    } else {                            \
        struct type *curelm = (head)->slh_first;        \
                                    \
        while (curelm->field.sle_next != (elm))         \
            curelm = curelm->field.sle_next;        \
        curelm->field.sle_next =                \
            curelm->field.sle_next->field.sle_next;     \
    }                               \
    _Q_INVALIDATE((elm)->field.sle_next);               \
} while (0)

/*
 * List definitions.
 */
#define LIST_HEAD(name, type)                       \
struct name {                               \
    struct type *lh_first;  /* first element */         \
}

#define LIST_HEAD_INITIALIZER(head)                 \
    { NULL }

#define LIST_ENTRY(type)                        \
struct {                                \
    struct type *le_next;   /* next element */          \
    struct type **le_prev;  /* address of previous next element */  \
}

/*
 * List access methods.
 */
#define LIST_FIRST(head)        ((head)->lh_first)
#define LIST_END(head)          NULL
#define LIST_EMPTY(head)        (LIST_FIRST(head) == LIST_END(head))
#define LIST_NEXT(elm, field)       ((elm)->field.le_next)

#define LIST_FOREACH(var, head, field)                  \
    for((var) = LIST_FIRST(head);                   \
        (var)!= LIST_END(head);                 \
        (var) = LIST_NEXT(var, field))

#define LIST_FOREACH_SAFE(var, head, field, tvar)           \
    for ((var) = LIST_FIRST(head);              \
        (var) && ((tvar) = LIST_NEXT(var, field), 1);       \
        (var) = (tvar))

/*
 * List functions.
 */
#define LIST_INIT(head) do {                        \
    LIST_FIRST(head) = LIST_END(head);              \
} while (0)

#define LIST_INSERT_AFTER(listelm, elm, field) do {         \
    if (((elm)->field.le_next = (listelm)->field.le_next) != NULL)  \
        (listelm)->field.le_next->field.le_prev =       \
            &(elm)->field.le_next;              \
    (listelm)->field.le_next = (elm);               \
    (elm)->field.le_prev = &(listelm)->field.le_next;       \
} while (0)

#define LIST_INSERT_BEFORE(listelm, elm, field) do {            \
    (elm)->field.le_prev = (listelm)->field.le_prev;        \
    (elm)->field.le_next = (listelm);               \
    *(listelm)->field.le_prev = (elm);              \
    (listelm)->field.le_prev = &(elm)->field.le_next;       \
} while (0)

#define LIST_INSERT_HEAD(head, elm, field) do {             \
    if (((elm)->field.le_next = (head)->lh_first) != NULL)      \
        (head)->lh_first->field.le_prev = &(elm)->field.le_next;\
    (head)->lh_first = (elm);                   \
    (elm)->field.le_prev = &(head)->lh_first;           \
} while (0)

#define LIST_REMOVE(elm, field) do {                    \
    if ((elm)->field.le_next != NULL)               \
        (elm)->field.le_next->field.le_prev =           \
            (elm)->field.le_prev;               \
    *(elm)->field.le_prev = (elm)->field.le_next;           \
    _Q_INVALIDATE((elm)->field.le_prev);                \
    _Q_INVALIDATE((elm)->field.le_next);                \
} while (0)

#define LIST_REPLACE(elm, elm2, field) do {             \
    if (((elm2)->field.le_next = (elm)->field.le_next) != NULL) \
        (elm2)->field.le_next->field.le_prev =          \
            &(elm2)->field.le_next;             \
    (elm2)->field.le_prev = (elm)->field.le_prev;           \
    *(elm2)->field.le_prev = (elm2);                \
    _Q_INVALIDATE((elm)->field.le_prev);                \
    _Q_INVALIDATE((elm)->field.le_next);                \
} while (0)

/*
 * Simple queue definitions.
 */
#define SIMPLEQ_HEAD(name, type)                    \
struct name {                               \
    struct type *sqh_first; /* first element */         \
    struct type **sqh_last; /* addr of last next element */     \
}

#define SIMPLEQ_HEAD_INITIALIZER(head)                  \
    { NULL, &(head).sqh_first }

#define SIMPLEQ_ENTRY(type)                     \
struct {                                \
    struct type *sqe_next;  /* next element */          \
}

/*
 * Simple queue access methods.
 */
#define SIMPLEQ_FIRST(head)     ((head)->sqh_first)
#define SIMPLEQ_END(head)       NULL
#define SIMPLEQ_EMPTY(head)     (SIMPLEQ_FIRST(head) == SIMPLEQ_END(head))
#define SIMPLEQ_NEXT(elm, field)    ((elm)->field.sqe_next)

#define SIMPLEQ_FOREACH(var, head, field)               \
    for((var) = SIMPLEQ_FIRST(head);                \
        (var) != SIMPLEQ_END(head);                 \
        (var) = SIMPLEQ_NEXT(var, field))

#define SIMPLEQ_FOREACH_SAFE(var, head, field, tvar)            \
    for ((var) = SIMPLEQ_FIRST(head);               \
        (var) && ((tvar) = SIMPLEQ_NEXT(var, field), 1);        \
        (var) = (tvar))

/*
 * Simple queue functions.
 */
#define SIMPLEQ_INIT(head) do {                     \
    (head)->sqh_first = NULL;                   \
    (head)->sqh_last = &(head)->sqh_first;              \
} while (0)

#define SIMPLEQ_INSERT_HEAD(head, elm, field) do {          \
    if (((elm)->field.sqe_next = (head)->sqh_first) == NULL)    \
        (head)->sqh_last = &(elm)->field.sqe_next;      \
    (head)->sqh_first = (elm);                  \
} while (0)

#define SIMPLEQ_INSERT_TAIL(head, elm, field) do {          \
    (elm)->field.sqe_next = NULL;                   \
    *(head)->sqh_last = (elm);                  \
    (head)->sqh_last = &(elm)->field.sqe_next;          \
} while (0)

#define SIMPLEQ_INSERT_AFTER(head, listelm, elm, field) do {        \
    if (((elm)->field.sqe_next = (listelm)->field.sqe_next) == NULL)\
        (head)->sqh_last = &(elm)->field.sqe_next;      \
    (listelm)->field.sqe_next = (elm);              \
} while (0)

#define SIMPLEQ_REMOVE_HEAD(head, field) do {           \
    if (((head)->sqh_first = (head)->sqh_first->field.sqe_next) == NULL) \
        (head)->sqh_last = &(head)->sqh_first;          \
} while (0)

#define SIMPLEQ_REMOVE_AFTER(head, elm, field) do {         \
    if (((elm)->field.sqe_next = (elm)->field.sqe_next->field.sqe_next) \
        == NULL)                            \
        (head)->sqh_last = &(elm)->field.sqe_next;      \
} while (0)

#define SIMPLEQ_CONCAT(head1, head2) do {               \
    if (!SIMPLEQ_EMPTY((head2))) {                  \
        *(head1)->sqh_last = (head2)->sqh_first;        \
        (head1)->sqh_last = (head2)->sqh_last;          \
        SIMPLEQ_INIT((head2));                  \
    }                               \
} while (0)

/*
 * XOR Simple queue definitions.
 */
#define XSIMPLEQ_HEAD(name, type)                   \
struct name {                               \
    struct type *sqx_first; /* first element */         \
    struct type **sqx_last; /* addr of last next element */     \
    unsigned long sqx_cookie;                   \
}

#define XSIMPLEQ_ENTRY(type)                        \
struct {                                \
    struct type *sqx_next;  /* next element */          \
}

/*
 * XOR Simple queue access methods.
 */
#define XSIMPLEQ_XOR(head, ptr)     ((__typeof(ptr))((head)->sqx_cookie ^ \
                    (unsigned long)(ptr)))
#define XSIMPLEQ_FIRST(head)        XSIMPLEQ_XOR(head, ((head)->sqx_first))
#define XSIMPLEQ_END(head)      NULL
#define XSIMPLEQ_EMPTY(head)        (XSIMPLEQ_FIRST(head) == XSIMPLEQ_END(head))
#define XSIMPLEQ_NEXT(head, elm, field)    XSIMPLEQ_XOR(head, ((elm)->field.sqx_next))


#define XSIMPLEQ_FOREACH(var, head, field)              \
    for ((var) = XSIMPLEQ_FIRST(head);              \
        (var) != XSIMPLEQ_END(head);                \
        (var) = XSIMPLEQ_NEXT(head, var, field))

#define XSIMPLEQ_FOREACH_SAFE(var, head, field, tvar)           \
    for ((var) = XSIMPLEQ_FIRST(head);              \
        (var) && ((tvar) = XSIMPLEQ_NEXT(head, var, field), 1); \
        (var) = (tvar))

/*
 * XOR Simple queue functions.
 */
#define XSIMPLEQ_INIT(head) do {                    \
    arc4random_buf(&(head)->sqx_cookie, sizeof((head)->sqx_cookie)); \
    (head)->sqx_first = XSIMPLEQ_XOR(head, NULL);           \
    (head)->sqx_last = XSIMPLEQ_XOR(head, &(head)->sqx_first);  \
} while (0)

#define XSIMPLEQ_INSERT_HEAD(head, elm, field) do {         \
    if (((elm)->field.sqx_next = (head)->sqx_first) ==      \
        XSIMPLEQ_XOR(head, NULL))                   \
        (head)->sqx_last = XSIMPLEQ_XOR(head, &(elm)->field.sqx_next); \
    (head)->sqx_first = XSIMPLEQ_XOR(head, (elm));          \
} while (0)

#define XSIMPLEQ_INSERT_TAIL(head, elm, field) do {         \
    (elm)->field.sqx_next = XSIMPLEQ_XOR(head, NULL);       \
    *(XSIMPLEQ_XOR(head, (head)->sqx_last)) = XSIMPLEQ_XOR(head, (elm)); \
    (head)->sqx_last = XSIMPLEQ_XOR(head, &(elm)->field.sqx_next);  \
} while (0)

#define XSIMPLEQ_INSERT_AFTER(head, listelm, elm, field) do {       \
    if (((elm)->field.sqx_next = (listelm)->field.sqx_next) ==  \
        XSIMPLEQ_XOR(head, NULL))                   \
        (head)->sqx_last = XSIMPLEQ_XOR(head, &(elm)->field.sqx_next); \
    (listelm)->field.sqx_next = XSIMPLEQ_XOR(head, (elm));      \
} while (0)

#define XSIMPLEQ_REMOVE_HEAD(head, field) do {              \
    if (((head)->sqx_first = XSIMPLEQ_XOR(head,         \
        (head)->sqx_first)->field.sqx_next) == XSIMPLEQ_XOR(head, NULL)) \
        (head)->sqx_last = XSIMPLEQ_XOR(head, &(head)->sqx_first); \
} while (0)

#define XSIMPLEQ_REMOVE_AFTER(head, elm, field) do {            \
    if (((elm)->field.sqx_next = XSIMPLEQ_XOR(head,         \
        (elm)->field.sqx_next)->field.sqx_next)         \
        == XSIMPLEQ_XOR(head, NULL))                \
        (head)->sqx_last =                  \
            XSIMPLEQ_XOR(head, &(elm)->field.sqx_next);     \
} while (0)


/*
 * Tail queue definitions.
 */
#define TAILQ_HEAD(name, type)                      \
struct name {                               \
    struct type *tqh_first; /* first element */         \
    struct type **tqh_last; /* addr of last next element */     \
}

#define TAILQ_HEAD_INITIALIZER(head)                    \
    { NULL, &(head).tqh_first }

#define TAILQ_ENTRY(type)                       \
struct {                                \
    struct type *tqe_next;  /* next element */          \
    struct type **tqe_prev; /* address of previous next element */  \
}

/*
 * Tail queue access methods.
 */
#define TAILQ_FIRST(head)       ((head)->tqh_first)
#define TAILQ_END(head)         NULL
#define TAILQ_NEXT(elm, field)      ((elm)->field.tqe_next)
#define TAILQ_LAST(head, headname)                  \
    (*(((struct headname *)((head)->tqh_last))->tqh_last))
/* XXX */
#define TAILQ_PREV(elm, headname, field)                \
    (*(((struct headname *)((elm)->field.tqe_prev))->tqh_last))
#define TAILQ_EMPTY(head)                       \
    (TAILQ_FIRST(head) == TAILQ_END(head))

#define TAILQ_FOREACH(var, head, field)                 \
    for((var) = TAILQ_FIRST(head);                  \
        (var) != TAILQ_END(head);                   \
        (var) = TAILQ_NEXT(var, field))

#define TAILQ_FOREACH_SAFE(var, head, field, tvar)          \
    for ((var) = TAILQ_FIRST(head);                 \
        (var) != TAILQ_END(head) &&                 \
        ((tvar) = TAILQ_NEXT(var, field), 1);           \
        (var) = (tvar))


#define TAILQ_FOREACH_REVERSE(var, head, headname, field)       \
    for((var) = TAILQ_LAST(head, headname);             \
        (var) != TAILQ_END(head);                   \
        (var) = TAILQ_PREV(var, headname, field))

#define TAILQ_FOREACH_REVERSE_SAFE(var, head, headname, field, tvar)    \
    for ((var) = TAILQ_LAST(head, headname);            \
        (var) != TAILQ_END(head) &&                 \
        ((tvar) = TAILQ_PREV(var, headname, field), 1);     \
        (var) = (tvar))

/*
 * Tail queue functions.
 */
#define TAILQ_INIT(head) do {                       \
    (head)->tqh_first = NULL;                   \
    (head)->tqh_last = &(head)->tqh_first;              \
} while (0)

#define TAILQ_INSERT_HEAD(head, elm, field) do {            \
    if (((elm)->field.tqe_next = (head)->tqh_first) != NULL)    \
        (head)->tqh_first->field.tqe_prev =         \
            &(elm)->field.tqe_next;             \
    else                                \
        (head)->tqh_last = &(elm)->field.tqe_next;      \
    (head)->tqh_first = (elm);                  \
    (elm)->field.tqe_prev = &(head)->tqh_first;         \
} while (0)

#define TAILQ_INSERT_TAIL(head, elm, field) do {            \
    (elm)->field.tqe_next = NULL;                   \
    (elm)->field.tqe_prev = (head)->tqh_last;           \
    *(head)->tqh_last = (elm);                  \
    (head)->tqh_last = &(elm)->field.tqe_next;          \
} while (0)

#define TAILQ_INSERT_AFTER(head, listelm, elm, field) do {      \
    if (((elm)->field.tqe_next = (listelm)->field.tqe_next) != NULL)\
        (elm)->field.tqe_next->field.tqe_prev =         \
            &(elm)->field.tqe_next;             \
    else                                \
        (head)->tqh_last = &(elm)->field.tqe_next;      \
    (listelm)->field.tqe_next = (elm);              \
    (elm)->field.tqe_prev = &(listelm)->field.tqe_next;     \
} while (0)

#define TAILQ_INSERT_BEFORE(listelm, elm, field) do {           \
    (elm)->field.tqe_prev = (listelm)->field.tqe_prev;      \
    (elm)->field.tqe_next = (listelm);              \
    *(listelm)->field.tqe_prev = (elm);             \
    (listelm)->field.tqe_prev = &(elm)->field.tqe_next;     \
} while (0)

#define TAILQ_REMOVE(head, elm, field) do {             \
    if (((elm)->field.tqe_next) != NULL)                \
        (elm)->field.tqe_next->field.tqe_prev =         \
            (elm)->field.tqe_prev;              \
    else                                \
        (head)->tqh_last = (elm)->field.tqe_prev;       \
    *(elm)->field.tqe_prev = (elm)->field.tqe_next;         \
    _Q_INVALIDATE((elm)->field.tqe_prev);               \
    _Q_INVALIDATE((elm)->field.tqe_next);               \
} while (0)

#define TAILQ_REPLACE(head, elm, elm2, field) do {          \
    if (((elm2)->field.tqe_next = (elm)->field.tqe_next) != NULL)   \
        (elm2)->field.tqe_next->field.tqe_prev =        \
            &(elm2)->field.tqe_next;                \
    else                                \
        (head)->tqh_last = &(elm2)->field.tqe_next;     \
    (elm2)->field.tqe_prev = (elm)->field.tqe_prev;         \
    *(elm2)->field.tqe_prev = (elm2);               \
    _Q_INVALIDATE((elm)->field.tqe_prev);               \
    _Q_INVALIDATE((elm)->field.tqe_next);               \
} while (0)

#define TAILQ_CONCAT(head1, head2, field) do {              \
    if (!TAILQ_EMPTY(head2)) {                  \
        *(head1)->tqh_last = (head2)->tqh_first;        \
        (head2)->tqh_first->field.tqe_prev = (head1)->tqh_last; \
        (head1)->tqh_last = (head2)->tqh_last;          \
        TAILQ_INIT((head2));                    \
    }                               \
} while (0)

/*
 * Singly-linked Tail queue declarations.
 */
#define STAILQ_HEAD(name, type)                     \
struct name {                               \
    struct type *stqh_first;    /* first element */     \
    struct type **stqh_last;    /* addr of last next element */ \
}

#define STAILQ_HEAD_INITIALIZER(head)                   \
    { NULL, &(head).stqh_first }

#define STAILQ_ENTRY(type)                      \
struct {                                \
    struct type *stqe_next; /* next element */          \
}

/*
 * Singly-linked Tail queue access methods.
 */
#define STAILQ_FIRST(head)  ((head)->stqh_first)
#define STAILQ_END(head)    NULL
#define STAILQ_EMPTY(head)  (STAILQ_FIRST(head) == STAILQ_END(head))
#define STAILQ_NEXT(elm, field) ((elm)->field.stqe_next)

#define STAILQ_FOREACH(var, head, field)                \
    for ((var) = STAILQ_FIRST(head);                \
        (var) != STAILQ_END(head);                  \
        (var) = STAILQ_NEXT(var, field))

#define STAILQ_FOREACH_SAFE(var, head, field, tvar)         \
    for ((var) = STAILQ_FIRST(head);                \
        (var) && ((tvar) = STAILQ_NEXT(var, field), 1);     \
        (var) = (tvar))

/*
 * Singly-linked Tail queue functions.
 */
#define STAILQ_INIT(head) do {                      \
    STAILQ_FIRST((head)) = NULL;                    \
    (head)->stqh_last = &STAILQ_FIRST((head));          \
} while (0)

#define STAILQ_INSERT_HEAD(head, elm, field) do {           \
    if ((STAILQ_NEXT((elm), field) = STAILQ_FIRST((head))) == NULL) \
        (head)->stqh_last = &STAILQ_NEXT((elm), field);     \
    STAILQ_FIRST((head)) = (elm);                   \
} while (0)

#define STAILQ_INSERT_TAIL(head, elm, field) do {           \
    STAILQ_NEXT((elm), field) = NULL;               \
    *(head)->stqh_last = (elm);                 \
    (head)->stqh_last = &STAILQ_NEXT((elm), field);         \
} while (0)

#define STAILQ_INSERT_AFTER(head, listelm, elm, field) do {     \
    if ((STAILQ_NEXT((elm), field) = STAILQ_NEXT((elm), field)) == NULL)\
        (head)->stqh_last = &STAILQ_NEXT((elm), field);     \
    STAILQ_NEXT((elm), field) = (elm);              \
} while (0)

#define STAILQ_REMOVE_HEAD(head, field) do {                            \
    if ((STAILQ_FIRST((head)) =                 \
        STAILQ_NEXT(STAILQ_FIRST((head)), field)) == NULL)      \
        (head)->stqh_last = &STAILQ_FIRST((head));      \
} while (0)

#define STAILQ_REMOVE_AFTER(head, elm, field) do {                      \
    if ((STAILQ_NEXT(elm, field) =                  \
        STAILQ_NEXT(STAILQ_NEXT(elm, field), field)) == NULL)   \
        (head)->stqh_last = &STAILQ_NEXT((elm), field);     \
} while (0)

#define STAILQ_REMOVE(head, elm, type, field) do {          \
    if (STAILQ_FIRST((head)) == (elm)) {                \
        STAILQ_REMOVE_HEAD((head), field);          \
    } else {                            \
        struct type *curelm = (head)->stqh_first;       \
        while (STAILQ_NEXT(curelm, field) != (elm))     \
            curelm = STAILQ_NEXT(curelm, field);        \
        STAILQ_REMOVE_AFTER(head, curelm, field);       \
    }                               \
} while (0)

#define STAILQ_CONCAT(head1, head2) do {                \
    if (!STAILQ_EMPTY((head2))) {                   \
        *(head1)->stqh_last = (head2)->stqh_first;      \
        (head1)->stqh_last = (head2)->stqh_last;        \
        STAILQ_INIT((head2));                   \
    }                               \
} while (0)

#define STAILQ_LAST(head, type, field)                  \
    (STAILQ_EMPTY((head)) ? NULL :                  \
            ((struct type *)(void *)                \
        ((char *)((head)->stqh_last) - offsetof(struct type, field))))

#endif  /* !_SYS_QUEUE_H_ */